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Chapter 13 Galaxies: Normal and Active. In this chapter, you will try to understand how galaxies form and evolve. You will discover that the amount of gas and dust in a galaxy is a critical clue.

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Chapter 13 Galaxies: Normal and Active

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Chapter 13Galaxies: Normal and Active

  • In this chapter, you will try to understand how galaxies form and evolve.

    • You will discover that the amount of gas and dust in a galaxy is a critical clue.

    • You will also discover that interactions between galaxies can influence their structure and evolution.

The Family of Galaxies

  • Astronomers classify galaxies according to shape—using a system developed in the 1920s by Edwin Hubble (namesake of the Hubble Space Telescope).

    • Creating a system of classification is a fundamental technique in science.

The Family of Galaxies

  • There are three important points to note about galaxy classification.

The Family of Galaxies

  • One, many galaxies have no disk, no spiral arms, and almost no gas and dust.

    • These elliptical galaxies range from huge giants to small dwarfs.

The Family of Galaxies

  • Two, disk-shaped galaxies usually have spiral arms and contain gas and dust, although some have very little.

    • Many spiral galaxies have a barred structure.

The Family of Galaxies

  • Three, some galaxies are highly irregular in shape and tend to be rich in gas and dust.

The Family of Galaxies

  • Spiral galaxies contain hot, bright stars—and are thus very luminous and easy to see.

  • Among spiral galaxies, about two-thirds are barred spirals.

    • From careful studies, astronomers can conclude that ellipticals are more common than spirals.

    • The irregulars make up only about 25 percent of all galaxies.

The Family of Galaxies

  • Different kinds of galaxies have different colors, depending mostly on how much star formation is happening in them.

The Family of Galaxies

  • Spirals and irregulars usually contain plenty of young stars, including massive, hot, luminous O and B stars.

    • They produce most of the light and give spirals and irregulars a distinct blue tint.

The Family of Galaxies

  • In contrast, elliptical galaxies usually contain few young stars.

  • The most luminous stars in ellipticals are red giants, which give those galaxies a red tint.

The Family of Galaxies

  • How many galaxies are there?

    • A research effort called GOODS (Great Observatories Origins Deep Survey) has used various telescopes.

    • These include the Hubble Space Telescope, the Chandra X-Ray Observatory, the Spitzer Space Telescope, and the XXM-Newton X-Ray Telescope.

    • Also there are the largest ground-based telescopes, to study two selected areas in the northern and southern skies.

The Family of Galaxies

  • The GOODS deep images reveal tremendous numbers of galaxies.

The Family of Galaxies

  • There are good reasons to believe that the two regions of the sky chosen for study are typical.

    • So, it seems likely that the entire sky is carpeted with galaxies.

    • At least, 100 billion would be visible if today’s telescopes were used for an all-sky census.

Measuring the Properties of Galaxies

  • Beyond the edge of Milky Way, astronomers find many billions of galaxies.

    • What are the properties of these star systems?

    • What are the diameters, luminosities, and masses of galaxies?


  • The distances to galaxies are so large that it is not convenient to express them in light-years, parsecs, or even kiloparsecs.

    • Instead, astronomers use the unit megaparsec (Mpc), or 1 million pc.

    • One Mpc equals 3.26 million ly, or approximately 3 x 1019 km (2 x 1019 miles).


  • To find the distance to a galaxy, astronomers must search among its stars, nebulae, and star clusters for familiar objects.

  • They must know the luminosity and diameter.

    • Such objects are called standard candles.

    • If you can find a standard candle in a galaxy, you can judge its distance.


  • When a supernova explodes in a distant galaxy, astronomers rush to observe it.

    • Studies show that type Ia supernovae—caused by the collapse of a white dwarf—all reach about the same absolute magnitude at maximum.

    • This makes them more like “standard bombs” than standard candles.


  • When type Ia supernovae are seen in more distant galaxies, astronomers can measure the apparent brightness at maximum.

    • They can compare that with the known luminosity of these supernovae to find their distances.


  • Astronomers use calibration to build a distance scale reaching from the nearest galaxies to the most distant visible galaxies.

    • Often, they refer to this as the distanceladder—as each step depends on the steps below it.


  • Of course, the foundation of the scale rests on understanding the luminosities of stars.

    • This ultimately rests on measurements of stellar parallax.

Telescopes as Time Machines

  • The most distant visible galaxies are little over 10 billion ly (3,000 Mpc) away.

  • When you look at a galaxy only a few million light-years away, you do not see it as it is now but as it was millions of years ago.

    • This is when its light began the journey towards Earth.

Telescopes as Time Machines

  • When you look at a more distant galaxy, you look back into the past by an amount called the look-back time.

    • This is the time in years equal to the distance to the galaxy in light-years.

Telescopes as Time Machines

  • The look-back time to nearby objects is usually not significant.

    • The look-back time to the moon is 1.3 seconds, and to the sun only 8 minutes.

    • To the nearest star, it is about 4 years.

Telescopes as Time Machines

  • The Andromeda Galaxy has a look-back time of about 2 million years—a mere eye blink in the lifetime of a galaxy.

  • When astronomers look at more distant galaxies though, the look-back time becomes an appreciable part of the age of the universe.

Telescopes as Time Machines

  • When astronomers observe the most distant visible galaxies, they are looking back over 10 billion years.

    • At that time, the universe may have been significantly different.

The Hubble Law

  • Astronomers find it difficult to measure the distance to a galaxy.

  • They often estimate such distances using a simple relationship.

    • Early in the 20th century, astronomers noticed that the lines in galaxy spectra were shifted slightly toward longer wavelengths—redshifts.

    • These redshifts imply that the galaxies are receding from Earth.

The Hubble Law

  • In 1929, the American astronomer Edwin Hubble published a graph that plotted the apparent velocities of recession versus distance for a number of galaxies.

    • The points in the graph fell along a straight line.

The Hubble Law

  • This relation between apparent velocity of recession and distance is known as the Hubble law.

    • The slope of the line is known as the Hubble constant, symbolized by H.

The Hubble Law

  • The Hubble law is important in astronomy for two reasons.

    • It is taken as evidence that the universe is expanding.

    • Astronomers use it to estimate the distance to galaxies.

The Hubble Law

  • The distance to a galaxy can be found by dividing its apparent velocity of recession by the Hubble constant.

The Hubble Law

  • This is a very useful calculation—because it is usually possible to obtain a spectrum of a galaxy and measure its redshift.

    • You can do this even if it is too far away to have observable standard candles.

    • Obviously, knowing the Hubble constant is important.

The Hubble Law

  • Edwin Hubble’s original measurement of H was too large because of errors in his measurements of the distances to galaxies.

    • Later astronomers have struggled to measure this important constant.

The Hubble Law

  • The most precise measurements of the Hubble constant yield a value of H about 70 km/s/Mpc. (This is with an uncertainty of about 5 percent.)

    • This means that a galaxy at a distance of 1 Mpc from the Milky Way is receding from us at a rate of 70 km/s.

    • A galaxy 2 Mpc away is receding at 140 km/s.

The Hubble Law

  • Note that modern astronomers interpret redshifts of galaxies as caused by the expansion of the universe.

    • The Hubble law allows them to estimate the distance to a galaxy from its redshift.

Galaxy Distances and Luminosities

  • The distance to a galaxy is the key to finding its diameter and its luminosity.

    • With even a modest telescope and a CCD camera, you could photograph a galaxy and measure its angular diameter.

    • If you knew the distance to the galaxy, you can then find its linear diameter.

    • Also, if you measure the apparent brightness of the galaxy, you can use the distance to find its luminosity.

Galaxy Distances and Luminosities

  • The results of such observations show that galaxies differ dramatically in size and luminosity.

    • Irregular galaxies tend to be small—1 to 25 percent the size of Milky Way—and have low luminosity.

    • Although they are common, they are easy to overlook.

Galaxy Distances and Luminosities

  • Our Milky Way is large and luminous compared with most spiral galaxies.

    • However, astronomers know of a few spiral galaxies that are even larger and more luminous.

Galaxy Distances and Luminosities

  • Elliptical galaxies cover a wide range of diameters and luminosities.

    • The largest, called giant ellipticals, are five times the size of Milky Way.

    • Many elliptical galaxies, though, dwarf ellipticals—are only 1 percent the diameter of our galaxy.

Galaxy Distances and Luminosities

  • Clearly, the diameter and luminosity of a galaxy do not determine its type.

    • Some small galaxies are irregular, and some are elliptical.

    • Some large galaxies are spiral, and some are elliptical.

    • Other factors must influence the origin and evolution of galaxies.

Galaxy Masses

  • The mass of a galaxy is difficult to determine.

  • Nevertheless, it is an important quantity.

    • It shows you how much matter the galaxy contains—which provides clues to the galaxy’s origin and evolution.

Galaxy Masses

  • The most precise method for measuring the mass of a galaxy is called the rotation curve method.

Galaxy Masses

  • The rotation curve method requires knowing:

    • The true sizes of the orbits of stars or gas clouds within a galaxy, which in turn requires knowing the distance of that galaxy

    • The orbital speeds of the stars or gas clouds, measured from the Doppler shifts of their spectral lines

Galaxy Masses

  • That is enough information to use Kepler’s third law and find the mass of the part of the galaxy contained within the star orbits with measured sizes and speeds.

Galaxy Masses

  • The rotation curve method works only for galaxies near enough to be well resolved.

    • More distant galaxies appear so small that astronomers cannot measure the radial velocity at different points across the galaxy.

    • They must use other, less precise, methods to estimate masses.

Galaxy Masses

  • The masses of galaxies cover a wide range.

    • The smallest contain about 10-6 as much mass as the Milky Way.

    • The largest contain as much as 50 times more than the Milky Way.

Dark Matter in Galaxies

  • Given the size and luminosity of a galaxy, astronomers can make a rough guess as to the amount of matter it should contain.

    • They know how much light stars produce.

    • They also know how much matter there is between the stars.

    • So, it should be possible to estimate very roughly, the mass of a galaxy from its luminosity.

Dark Matter in Galaxies

  • When astronomers measure the masses of galaxies, however, they find that the measured masses are much larger than expected from the luminosities of the galaxies.

Dark Matter in Galaxies

  • Astronomer Vera Rubin found in the 1960s that this also seems to be true of most nearby galaxies.

    • Measured masses of galaxies amount to 10 to 100 times more mass than you can see.

Dark Matter in Galaxies

  • Dark matter is difficult to detect, and it is even harder to explain.

    • Some astronomers have suggested that dark matter consists of low-luminosity white dwarfs and brown dwarfs.

    • They are scattered through the halos of galaxies.

Dark Matter in Galaxies

  • Searches for white dwarfs and brown dwarfs in the halo of our galaxy have found a few, but not enough to make up most of the dark matter.

Dark Matter in Galaxies

  • The dark matter can’t be hidden in vast numbers of black holes and neutron stars.

    • Astronomers don’t see the X rays these objects would emit.

    • There is 10 to 100 times more dark matter than visible matter in galaxies.

    • That many black holes would produce X rays that would be easy to detect.

Dark Matter in Galaxies

  • Also, recent images from the Chandra X-ray observatory indicate:

    • A collision between two galaxy clusters caused their gas and dark matter components to separate.

Dark Matter in Galaxies

  • As observations imply, the dark matter can’t be composed of familiar objects or material.

    • Astronomers are forced to conclude that the dark matter is made up of unexpected forms of matter.

Dark Matter in Galaxies

  • Until recently, neutrinos were thought to be massless.

    • Studies now suggest they have a very small mass.

    • Thus, they may represent part of the dark matter.

Dark Matter in Galaxies

  • However, their masses are too low to make up all the dark matter.

    • There must be some other undiscovered form of matter in the universe that is detectable only by its gravitational field.

Dark Matter in Galaxies

  • Dark matter remains one of the fundamental unresolved problems of modern astronomy.

    • Observations of galaxies and clusters of galaxies reveal that 90 to 95 percent of the matter in the universe is dark matter.

Supermassive Black Holes in Galaxies

  • Doppler shift measurements show that the stars near the centers of many galaxies are orbiting very rapidly.

Supermassive Black Holes in Galaxies

  • To hold stars in such small, short-period orbits, the centers of galaxies must contain masses of a million to a few billion solar masses.

    • Yet, no object is visible.

    • The evidence seems to require that the nuclei of galaxies contain supermassive black holes.

    • You have learned that Milky Way contains a supermassive black hole at its center.

    • Evidently, that is typical of galaxies.

Supermassive Black Holes in Galaxies

  • It is a common misconception that the orbits of stars throughout a galaxy are controlled by the central black hole.

    • The masses of those black holes, large as they may seem, are negligible compared with a galaxy’s mass.

    • The 2.6-million-solar-mass black hole at the center of Milky Way contains only a thousandth of one per cent of the total mass of the galaxy.

The Evolution of Galaxies

  • Your goal in the chapter has been to build a theory that explains the evolution of galaxies.

    • Earlier, you developed a theory that described the origin of Milky Way.

    • Presumably, other galaxies formed in similar ways.

The Evolution of Galaxies

  • Why did some galaxies become spiral, some elliptical, and some irregular?

    • Clues to that mystery lie in the clustering of galaxies.

Clusters of Galaxies

  • The distribution of galaxies is not entirely random.

    • Galaxies tend to occur in clusters ranging from a few to thousands.

    • Deep photos made with the largest telescopes reveal clusters of galaxies scattered out to the limits of visibility.

    • This clustering of the galaxies can help you understand their evolution.

Clusters of Galaxies

  • For this discussion, you can sort clusters into two groups—rich and poor.

  • Rich galaxy clusters contain over a thousand galaxies, mostly elliptical.

    • They are scattered through a spherical volume about 3 Mpc (107 ly) in diameter.

Clusters of Galaxies

  • The Coma cluster (located 100 Mpc from Earth in the direction of the constellation Coma Berenices) is an example of a rich cluster.

    • It contains at least 1,000 galaxies, mostly E and S0 types.

    • Close to its center area giant elliptical galaxy and a large S0 galaxy.

Clusters of Galaxies

  • Rich clusters often contain one or more giant elliptical galaxies at their centers.

  • In contrast, poor galaxy clusters contain fewer than 1,000 galaxies.

    • They are irregularly shaped, and are less crowded towards the center.

Clusters of Galaxies

  • Our own Local Group, which contains Milky Way, is a good example of a poor cluster.

Clusters of Galaxies

  • It contains a few dozen members scattered irregularly through a volume slightly over 1 Mpc in diameter.

Clusters of Galaxies

  • Of the brighter galaxies, 14 are elliptical, 3 are spiral, and 4 are irregular.

Clusters of Galaxies

  • Classifying clusters as either rich or poor reveals a fascinating and suggestive clue to the evolution of galaxies.

    • In general, rich clusters tend to contain 80 to 90 percent E and S0 galaxies and few spirals.

    • Poor clusters contain a larger percentage of spirals.

    • Among isolated galaxies that are not in clusters, 80 to 90 percent are spirals.

Clusters of Galaxies

  • This suggests that a galaxy’s environment is important in determining its structure.

    • This has led astronomers to suspect that the secrets of galaxy evolution lie in collisions between galaxies.

Colliding Galaxies

  • Astronomers are finding more and more evidence to show that galaxies collide, interact, and merge.

    • In fact, collisions among galaxies may dominate their evolution.

Colliding Galaxies

  • You should not be surprised that galaxies collide with each other.

    • The average separation between galaxies is only about 20 times their diameter.

    • So, astronomically speaking, galaxies should bump into each other fairly often.

Colliding Galaxies

  • Stars, on the other hand, almost never collide—because the typical separation between stars is about 107 times their diameter.

    • A collision between two stars is about as likely as a collision between two gnats flitting about in a baseball stadium.

Colliding Galaxies

  • There are several important points to note about interacting galaxies.

Colliding Galaxies

  • One, interacting galaxies can distort each other with tides—producing tidal tails and shells of stars.

    • They may even trigger the formation of spiral arms.

    • Large galaxies can even absorb smaller galaxies.

Colliding Galaxies

  • Two, the interactions can trigger star formation.

Colliding Galaxies

  • Three, evidence left inside galaxies in the form of motions and multiple nuclei reveals that they have suffered past interactions and mergers.

Colliding Galaxies

  • Four, the beautiful ring galaxies are bull’s-eyes left behind by high-speed collisions.

Colliding Galaxies

  • Evidence of galaxy mergers is all around.

    • Milky Way is a cannibal galaxy—snacking on the Magellanic Clouds as they orbit around it.

    • Its tides are pulling apart the Sagittarius and the Canis Major Dwarf galaxies.

    • This produces great streamers of stars wrapped around Milky Way.

    • Almost certainly, our galaxy has dined on other small galaxies in the past.

The Origin and Evolution of Galaxies

  • The test of any scientific understanding is whether you can put all the evidence and theory together to tell the history of the objects studied.

The Origin and Evolution of Galaxies

  • Can you describe the origin and evolution of the galaxies?

    • Just a few decades ago, it would have been impossible to do so.

    • However, the evidence from space telescopes and new-generation telescopes on Earth—combined with advances in computer modeling and theory—allow astronomers to outline the story of the galaxies.

The Origin and Evolution of Galaxies

  • Elliptical galaxies appear to be the product of galaxy mergers—which triggered star formation and used up all the gas and dust.

    • Astronomers see star formation being stimulated to high levels in many galaxies.

The Origin and Evolution of Galaxies

  • Starburst galaxies are very luminous in the infrared.

    • A collision has triggered bursts of star formation that are heating the dust.

    • The warm dust reradiates the energy in the infrared.

The Origin and Evolution of Galaxies

  • The Antennae contain over 15 billion solar masses of hydrogen gas.

    • It will become a starburst galaxy as their ongoing merger triggers rapid star formation.

The Origin and Evolution of Galaxies

  • A few collisions and mergers could leave a galaxy with no gas and dust from which to make new stars.

The Origin and Evolution of Galaxies

  • Astronomers now suspect that most ellipticals are formed by the merger of at least two or more galaxies:

    • Dwarf ellipticals, which are too small to be formed by mergers

    • Irregulars, which may be fragments left over by the merger of larger galaxies

The Origin and Evolution of Galaxies

  • In contrast, spirals seem never to have suffered major collisions.

    • Their thin disks are delicate and would be destroyed by tidal forces in a collision with a massive galaxy.

    • Also, they retain plenty of gas and dust and continue making stars.

The Origin and Evolution of Galaxies

  • Our Milky Way has, evidently, never merged with another large galaxy.

  • It has, however, a large spiral neighbor, the Andromeda Galaxy.

    • Both seem to have cannibalized smaller galaxies.

The Origin and Evolution of Galaxies

  • Barred spiral galaxies may be the product of tidal interactions.

    • Mathematical models show that the bars are not stable and eventually dissipate.

    • It may take tidal interactions with other galaxies to regenerate the bars.

    • As well over half of all spiral galaxies have bars, you can suspect that these tidal interactions are common.

The Origin and Evolution of Galaxies

  • Milky Way is probably a barred spiral.

    • This may be the result of the interaction with its two Magellanic Cloud companions, or the more distant but very massive Andromeda Galaxy.

The Origin and Evolution of Galaxies

  • Other processes can alter galaxies.

    • The S0 galaxies have disks and bulges like a spiral galaxy but no spiral arms.

    • They may have lost much of their gas and dust moving through the gas trapped in the dense clusters to which they belong.

The Origin and Evolution of Galaxies

  • For example, X-ray observations show that the Coma cluster contains thin, hot gas between the galaxies.

The Origin and Evolution of Galaxies

  • A galaxy moving through that gas would encounter a tremendous wind that could strip away its gas and dust.

The Origin and Evolution of Galaxies

  • Observations made with the largest and most sophisticated telescopes are taking astronomers back to the age of galaxy formation.

    • At great distances, the look-back time is so large that they see the universe as it was soon after galaxies began to form.

    • There were more spirals and fewer ellipticals.

The Origin and Evolution of Galaxies

  • The observations show that galaxies then were closer together.

    • About 33 percent of all distant galaxies are in close pairs.

    • However, only 7 percent of nearby galaxies are in pairs.

    • The observational evidence clearly supports the hypothesis that galaxies have evolved by merger.

The Origin and Evolution of Galaxies

  • The evolution of galaxies is not a simple process.

    • A good theory helps you understand how nature works.

    • Astronomers are just beginning to understand the exciting and complex story of the galaxies.

    • Nevertheless, it is already clear that galaxy evolution is much like a pie-throwing contest—and just about as neat.

Active Galaxies and Quasars

  • Many galaxies have powerful energy sources in their nuclei that, in some cases, produce powerful jets and other outbursts.

    • These are called activegalaxies.

Active Galaxies and Quasars

  • By looking far away and back in time, astronomers have discovered:

    • The origin of active galaxy energy sources and outbursts were closely related to the formation and history of galaxies.

Active Galaxies and Quasars

  • The first type of active galaxy was discovered in the 1950s.

    • They were named radio galaxiesbecause these galaxies are sources of unusually strong radio waves.

Active Galaxies and Quasars

  • By the 1970s, astronomers had put space telescopes in orbit.

    • They discovered that radio galaxies are generally bright at many other wavelengths.

Active Galaxies and Quasars

  • The flood of energy pouring out of active galaxies originates almost entirely in their nuclei.

    • They are referred to as active galactic nuclei (AGN).

Seyfert Galaxies

  • In 1943, astronomer Carl K. Seyfert conducted a study of spiral galaxies.

    • He noted that about two percent of spirals have small, highly luminous nuclei in their bulges.

Seyfert Galaxies

  • Today, these Seyfert galaxiesare recognized by the peculiar spectra of these luminous nuclei.

    • They contain broad emission lines of highly ionized atoms.

    • Emission lines suggest a hot, low-density gas.

    • The presence of ionized atoms is evidence that the gas is very hot.

    • The broad spectral lines indicate large Doppler shifts produced by high gas velocities.

Seyfert Galaxies

  • The velocities at the center of Seyfert galaxies are roughly 10,000 km/s.

  • This is about 30 times greater than velocities at the center of normal galaxies.

    • Something violent is happening in the cores of Seyfert galaxies.

Seyfert Galaxies

  • Astronomers later discovered that the brilliant nuclei of Seyfert galaxies change brightness rapidly.

    • This happens in only a few hours or minutes, especially at X-ray wavelengths.

Seyfert Galaxies

  • You have learned that an astronomical body cannot change its brightness significantly in a time shorter than the time it takes light to cross its diameter.

    • If the Seyfert nucleus can change in a few minutes, then it cannot be larger in diameter than a few light-minutes.

    • For comparison, the distance from Earth to the sun is 8 light minutes.

Seyfert Galaxies

  • Despite their small size, the brightest Seyfert nuclei emit a hundred times more energy than the entire Milky Way.

    • Something in the centers of these galaxies not much bigger than Earth’s orbit produces a galaxy’s worth of energy.

Seyfert Galaxies

  • Seyfert nuclei are three times more common in interacting pairs of galaxies than in isolated galaxies.

    • Also, about 25 percent have peculiar shapes—suggesting tidal interactions with other galaxies.

Seyfert Galaxies

  • This statistical evidence hints that Seyfert galaxies may have been triggered into activity by collisions or interactions with companions.

Seyfert Galaxies

  • Some Seyferts are expelling matter in oppositely directed jets.

    • You have learned about this geometry on smaller scales when matter flows into neutron stars and black holes and forms an accretion disk.

Seyfert Galaxies

  • All this evidence leads modern astronomers to conclude that the cores of Seyfert galaxies contain supermassive black holes.

    • This is a black hole with a mass as high as a billion solar masses, plus a correspondingly large accretion disk.

Seyfert Galaxies

  • The gas in the centers of Seyfert galaxies is traveling so fast it would escape from a normal galaxy.

    • Only very large, central masses could exert enough gravity to hold the gas inside the nuclei.

    • Encounters with other galaxies could throw matter into the black hole.

    • As you have learned, lots of energy can be liberated by matter flowing into a black hole.

Seyfert Galaxies

  • You have learned that Milky Way contains a massive central black hole.

    • However, the black hole seems to be on a starvation diet and is therefore relatively inactive.

Double-Lobed Radio Sources

  • Beginning in the 1950s, radio astronomers found that some sources of radio energy in the sky consisted of pairs of radio-bright regions.

    • When optical telescopes studied the locations of these double-lobed radio sources, they revealed galaxies located between the two lobes.

Double-Lobed Radio Sources

  • The geometry suggests that radio lobes are inflated by jets of excited gas emerging in two directions from the central galaxy.

    • Statistical evidence indicates that jets and radio lobes, like Seyfert nuclei, are associated with interacting galaxies.

Double-Lobed Radio Sources

  • The AGN jets seem to be related to matter falling into a central black hole via an accretion disk.

    • However, the details of this process are not understood.

Double-Lobed Radio Sources

  • The violence of these active galaxies is so great it can influence entire clusters of galaxies.

  • The Perseus galaxy cluster contains thousands of galaxies and is one of the largest objects in the universe.

Double-Lobed Radio Sources

  • One of its galaxies, NGC 1275, is among the largest galaxies known.

  • It is pumping out jets of high-energy particles, heating the gas in the galaxy cluster, and inflating

    low-density bubbles

    that distort the huge

    gas cloud.

Double-Lobed Radio Sources

  • The hot gas observed in galaxy clusters is heated to multimillion-degree temperatures as galaxy after galaxy goes through eruptive stages that can last for hundreds of millions of years.

  • NGC 1275 is erupting now, and it is so powerful and has heated the surrounding gas to such high temperatures that the gas can no longer fall in and the galaxy has probably limited its own growth.


  • The largest telescopes detect multitudes of faint points of light with peculiar emission spectra.

    • These objects are called quasars—also known as quasi-stellar objects (QSOs).


  • Astronomers now recognize quasars as extreme examples of AGN as well as some of the most distant visible objects in the universe.

    • However, they were a mystery when they were first identified.


  • In the early 1960s, photos of the location of some radio sources that resembled radio galaxies revealed only single starlike points of light.

    • The first of these objects to be identified was 3C48.

    • Later, the source 3C273 was found.


  • They were obviously not normal radio galaxies.

    • Even the most distant photographable galaxies look fuzzy.

    • These objects, however, looked like stars.

    • Their spectra, though, were totally unlike stellar spectra.

    • So, the objects were called quasi-stellar objects.


  • For a few years, the spectra of quasars were a mystery.

    • A few unidentifiable emission lines were superimposed on a continuous spectrum.


  • In 1963, astronomer Maarten Schmidt calculated that, if hydrogen Balmer lines were redshifted by z = 0.158, they would fit the observed lines in 3C273’s spectrum.


  • Other quasar spectra quickly yielded to this approach—revealing even larger redshifts.

    • To understand the significance of these large redshifts and the large velocities of recession they imply, you must recall the Hubble law.


  • The large redshifts of the quasars imply that they must be at great distances, some farther away than any known galaxy.

    • Many quasars are evidently so far away that galaxies at those distances are very difficult to detect.


  • Yet, the quasars are easily photographed.

    • Thus, quasars must have 10 to 1,000 times the luminosity of a large galaxy.

    • They must be ultraluminous.


  • Soon after quasars were discovered, astronomers detected fluctuations in their brightnesses over times as short as a few hours or minutes.

    • The rapid fluctuations showed that they are small objects like AGN, only a few light-minutes or light-hours in diameter.


  • Evidence has accumulated that quasars are the most luminous AGN, located in very distant galaxies.

    • For example, some quasars, like AGN, are at the centers of double radio lobes plus jets.


  • Perhaps, you have another question about quasar distances at this point.

  • How can astronomers be sure that quasars really are that far away?

    • Astronomers faced with explaining how a small object could produce so much energy asked themselves the same question.


  • In the early 1980s, astronomers were able to photograph faint nebulosity surrounding some quasars.

  • This was called quasar fuzz.

    • The spectra of quasar fuzz looked like the spectra of normal but very distant galaxies with the same redshift as the central quasar.


  • In other cases, quasar light shines through the outskirts of a distant galaxy.

    • The quasar spectrum has extra absorption lines from gas at the redshift of the galaxy.

    • They are smaller than the quasar’s redshift.

    • This means the quasar is farther away than the galaxy containing the gas that absorbed some of the quasar’s light on its way to Earth.


  • Both these observations confirm that galaxy and quasar redshifts indicate distances in a mutually consistent way.

The Search for a Unified Model

  • Astronomers studying galaxies are now developing a unified model of AGN and quasars.

    • A monster black hole is the centerpiece.

The Search for a Unified Model

  • Even a supermassive black hole is quite small compared with the galaxy.

    • A ten-million-solar-mass black hole would be only one-fifth the diameter of Earth’s orbit.

    • That means matter in an accretion disk can get very close to the black hole, orbit very fast, and grow very hot.

The Search for a Unified Model

  • Theoretical calculations indicate that the disk immediately around the black hole is ‘puffed up.’

    • It is also thick enough to

      hide the central black hole

      from some viewing angles.

The Search for a Unified Model

  • The hot inner disk seems to be the source of the jets often seen coming out of active galaxy cores.

    • However, the process by

      which jets are generated

      is not understood.

The Search for a Unified Model

  • The outer part of the disk—according to calculations—is a fat, cold torus (doughnut shape) of dusty gas.

The Search for a Unified Model

  • According to the unified model, what you see when you view the core of an AGN and QSO depends on how this accretion disk is tipped with respect

    to your line of sight.

The Search for a Unified Model

  • If you view the accretion disk from the edge, you cannot see the central zone.

    • The thick dusty torus blocks your view.

The Search for a Unified Model

  • Instead, you see radiation emitted by gas lying above and below the central disk.

    • It is therefore relatively


The Search for a Unified Model

  • Also, it moves relatively slowly, with small Doppler shifts.

    • Thus, you see narrower spectral lines coming from this narrow line region.

The Search for a Unified Model

  • If the accretion disk is tipped slightly, you may be able to see some of the intensely hot gas in the central cavity.

    • This broad line region emits broad spectral lines because the gas is so hot.

The Search for a Unified Model

  • It is orbiting at high velocities.

    • The resulting high Doppler shifts spread out the spectral line.

The Search for a Unified Model

  • If you look directly into the central cavity around the black hole, down the dragon’s throat, you see:

    • A jet emerging perpendicular to the accretion disk and coming straight at you

The Search for a Unified Model

  • Model calculations indicate this would result in a very bright and highly variable source with few or no emission lines.

    • This is the appearance of AGN known as blazars.

The Search for a Unified Model

  • Astronomers are now using this unified model to sort out the different kinds of active galaxies and quasars—so, they can understand how they are related.

The Search for a Unified Model

  • For example, about one percent of quasars are strong radio sources.

    • The radio radiation may come from synchrotron radiation produced in the high-energy gas and magnetic fields in the jets.

The Search for a Unified Model

  • Here’s another example.

    • Using infrared cameras to see through dust, astronomers observed the core of the double-lobed radio galaxy Cygnus A.

    • They found an object much like a quasar.

The Search for a Unified Model

  • Astronomers have begun to refer to such hidden objects as “buried quasars.”

The Search for a Unified Model

  • The unified model is far from complete.

    • The detailed structure of accretion disks is poorly understood—as is the process by which the disks produce jets.

    • Furthermore, the spiral Seyfert galaxies are clearly different from the giant elliptical galaxies that have double radio lobes.

The Search for a Unified Model

  • Unification does not explain all the differences among active galaxies.

    • It is a model that provides some clues to what is happening in AGN and quasars.

Origin of Supermassive Black Holes

  • Naturally, you are wondering where these supermassive black holes came from.

  • That question is linked to a second question.

  • What makes a supermassve black hole erupt?

    • Answering those questions will help you understand how galaxies form.

Origin of Supermassive Black Holes

  • Evidence is accumulating that most galaxies seem to contain a supermassive black hole at their center.

    • Even Milky Way and the nearby Andromeda Galaxy contain central black holes.

Origin of Supermassive Black Holes

  • Only a few percent of galaxies, however, have AGN.

    • That must mean that most of the supermassive black holes are dormant.

    • Presumably, they are not being fed large amounts of matter.

Origin of Supermassive Black Holes

  • A slow trickle of matter flowing into the supermassive black hole at the center of our galaxy could explain the mild activity seen there.

    • However, it would take a larger meal to trigger an eruption like those seen in AGN.

Origin of Supermassive Black Holes

  • What could trigger a supermassive black hole to erupt?

  • The answer is something you have learned earlier—tides.

    • You have learned how tides twist interacting galaxies and rip matter away into tidal tails.

Origin of Supermassive Black Holes

  • Active galaxies are often distorted.

    • They have evidently been twisted by tidal forces as they interacted or merged with another galaxy.

    • Mathematical models show that those interactions can also throw stars plus clouds of interstellar gas and dust inward toward the galaxies’ centers.

Origin of Supermassive Black Holes

  • A sudden flood of matter flowing into a supermassive black hole would trigger it into eruption.

Origin of Supermassive Black Holes

  • The figure shows how a passing star would be shredded andpartially consumed by a supermassive hole.

    • A steady diet of inflowing gas, dust, and an occasional star would keep an AGN powered by a supermassive black hole active.

Origin of Supermassive Black Holes

  • A few dozen supermassive black holes have been measured for their masses.

  • Their masses are correlated with the masses of the host galaxies’ nuclear bulges.

    • In each case, the mass of the black hole is about 0.5 percent the mass of the surrounding nuclear bulge.

Origin of Supermassive Black Holes

  • Apparently, a galaxy forms its nuclear bulge.

  • A certain fraction of the mass sinks to the center where it forms a supermassive black hole.

    • All that matter flowing together to form the black hole would release a tremendous amount of energy and trigger a violent eruption.

Origin of Supermassive Black Holes

  • Long ago, when galaxies were actively forming, the birth of the nuclear bulges must have triggered many AGN.

  • Later episodes of AGN activity could be triggered by interactions or mergers with other galaxies.

Quasars Through Time

  • The universe began 13.7 billion years ago.

  • Some quasars are over 10 billion light-years away.

    • Due to their large look-back times, they appear as they were when the universe was only 10 percent of its present age.

Quasars Through Time

  • The first clouds of gas that formed galaxies would have also made supermassive black holes at the centers of those galaxies’ nuclear bulges.

    • The abundance of matter flooding into these black holes could have triggered outbursts that are seen as quasars.

Quasars Through Time

  • Galaxies were closer together when the universe was young and had not expanded very much.

    • As they were closer together, the forming galaxies collided more often.

    • You have learned how collisions between galaxies could throw matter into supermassive black holes and trigger eruptions.

Quasars Through Time

  • Quasars are often located in host galaxies that are distorted as if they were interacting with other galaxies.

Quasars Through Time

  • Quasars are most common with redshifts of a little over 2 and less common with redshifts above 2.7.

  • The largest quasar redshifts are over 6.

    • Such high-redshift quasars are quite rare.

Quasars Through Time

  • Evidently, if you looked at quasars with redshifts of a little over 2, you would be looking back to an age when galaxies were actively forming, colliding, and merging.

    • In that age of quasars, they were about 1,000 times more common than they are now.

    • Even so, only a fraction of galaxies had quasars erupting in their cores at any one time.

Quasars Through Time

  • If you looked back to even higher redshifts, you would see fewer quasars.

    • You would be looking back to an age when the universe was so young that it had not yet begun to form many galaxies and quasars.

Quasars Through Time

  • Then, where are all the dead quasars?

    • There is no way to get rid of supermassive black holes.

Quasars Through Time

  • Astronomers have discovered that nearly all galaxies contain supermassive black holes.

    • Those black holes may have suffered quasar eruptions when the universe was younger, galaxies were closer together, and infalling gas and dust were more plentiful.

Quasars Through Time

  • However, quasar eruptions became less common as galaxies became more stable and as the abundance of gas and dust in the centers of galaxies were exhausted.

Quasars Through Time

  • Our own Milky Way is a good example.

    • It could have been a quasar long ago.

    • Today, though, its supermassive black hole is resting.

Quasars Through Time

  • Dormant black holes at the centers of galaxies today can be reawakened to become AGN by galaxy collisions.

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